The present invention relates generally to scrambling systems and methods, and more particularly, to an analog privacy scrambling system and method that scrambles analog signals such that a casual receiver cannot recover the analog information contained in the signal.
Historically, privacy for signals is established by digitizing the signals, then scrambling bits of the signal with a noise sequence to generate a transmitted signal. The modulation by the noise sequence is removed at the receiver resulting in a digital signal that is a replica of the original digitized signal. If an analog signal is desired, the signal may be passed through an digital-to-analog converter to recover the analog signal.
When the bandwidth to be transmitted is high enough, this process is not possible. Conversion of the signal to digital form is difficult for high bandwidth signals. If the conversion to digital format were possible, the transmission of the digital format requires several times the bandwidth of the original analog signal. Several bits are transmitted for each sample of the input signal, resulting in bit rates for communication that are typically more than 16 times the bandwidth of the signal. For example, transmission of a 1 GHz bandwidth with a 2 giga-sample per second sample rate, each sample with 8 bits per sample, requires a transmission bit rate of 16 giga-bits per second. Scrambling of these high data rates and the transmission bandwidth required, even for sophisticated modulation techniques, are very difficult.
Even when the bandwidth is not high, privacy for analog signals is difficult. If the dynamic range is high, conversion to a digital format can generate many bits of data, as discussed above. If an analog technique is to be used, the dynamic range should be preserved through the privacy processing. The privacy processing cannot introduce noise that reduces the dynamic range of the signal by an unacceptable amount.
Analog scrambling schemes have included inverting the spectrum of the signal at regular time intervals. The random inversion of the signal frequencies obscures the individual signals in the bandwidth. However, the privacy afforded by this technique is limited since the possible inversion of the signal bandwidth is performed at regular intervals. Processing of the signals with a Fourier transform can identify the time of the transition from the upright signal to the inverted signal and back. In addition the transitions generate transients in the signal that interfere with the quality of the signal after it has been reconstructed.
It is therefore an objective of the present invention to provide for analog privacy scrambling systems and methods that improves upon the above-discussed techniques.